NVIDIA Sponsorship, Launch Permits, Flying the Quad

NVIDIA is Armadillo Aerospace’s official sponsor for the X-Prize Cup events this year.

I was very reticent to talk sponsorship with any of the companies that I work with in my capacity at Id Software, because I didn’t want there to be any hint that there is a sense of obligation involved (either way) based on our working relationships in software, but I casually mentioned the possibility to NVIDIA a little while ago, and they jumped on it with a “We would love to be involved with that!”

This works out well, because I wasn’t sure if it would be worth it to me to put random sponsor logos all over our program just to defray costs. NVIDIA is a company that I have worked closely with for a long time, and I have wholeheartedly recommended their products on technical merit since the very early days (if we skip NV1 and RIVA128…). I’ll be happy to have our vehicles flying NVIDIA colors this year.

There is a chance at this point that I may have written the last personal check I need to for Armadillo. This sponsorship will cover all of our expenses through the X-Prize Cup, and I expect to come away from that with some prize money. If we run the table with first prize wins in both categories, we will be doing great. It still wouldn’t cover all the money I have put in over the last five years, but it should be plenty to take us through the commercialization of the current technologies. We have several customers ready to pay for flights even below the “man to 100km” goal of the suborbital space tourism market, so I don’t think we are too far off. If we don’t pocket the big prize, we will still carry on at our current pace, but if we get the extra money we will be stepping things up by bringing more of the team on full time.

Launch Permits

We have two experimental launch permit applications (the VDR and the Quad) in process with AST, both of which have been deemed “substantially complete”. While the legally mandated time period for official response is actually beyond the date of the XPC, AST understands the importance of the event, and is making all efforts to have things fully approved in time. There are other teams that have submitted initial applications, but we are the first organization to ever reach this milestone in the new experimental permit regime. This particular submission is specifically for the VTVL flights in front of the XPC crowds, and we are being run through the wringer even more than we would be for flights all the way to 100km in desolate areas. After the Cup event, we will be submitting modified permit applications for the envelope expansion flights to higher altitudes, then moving on to full launch license applications for commercial applications.

Neil Milburn has been working full time on this for a while. His day job is as a physics teacher, so we decided that it would be worth while to have him become Armadillo’s first full time (although temporary) employee instead of teaching a summer session this year, primarily to concentrate on the regulatory work. This doesn’t qualify as “fun” by any stretch of the imagination, but it is vital for our progress moving forward.

Flying the Quad

The quad has been up in the air four times this last month, as we sort out the issues involved in the new vehicle.

I finally got around to making an o-ring sealed leak testing plate that we can bolt to the bottom of the engine to allow us to leak check all the post-valve connections on the engine chamber: main fuel, main lox, igniter fuel, igniter lox, spark plug, chamber pressure port, chamber-to-injector. Lox leaks aren’t particularly troublesome, but fuel leaks can cause fires, and hot gas leaks can cause catastrophic burn through. The one problem with this is that the existing chamber retaining flange flexes a little bit as it is torqued down, causing the tie rods to splay out a bit, making it difficult to fit the leak test sealing plate on afterwards. I need to make a thicker bottom flange to avoid this in the future.

We intended to use 1.5″ diameter stainless flex hose for the main propellant lines, but it really wasn’t flexible enough. One axis is fine, but if it needs to twist at all, it is far too stiff. We went back to 1″ lines, which may be a bit of a performance limiter. The valves are 3/4″.

Ignition Tests
We had originally mounted the valves on the vehicle, and ran the flex lines directly to the engine, as we did with last year’s X-Prize Cup vehicle, but when we did ignition tests on the vehicle we got really messy flamey shutdowns as the purge threw out a quart or so of ethanol. We made some 45 degree adapters that allowed us to mount the valves back on the engine, which helped a lot.

Quad Hop1
When we set up for doing the first hover test, we became aware that there were a lot of things on the vehicle that the tethers could catch on. We had a couple bad instances of that last year, so we quickly threw together a crude vehicle cover out of spare parts around the shop. We still have bungee cords on the tethers to keep them from drooping for the first several feet, but once you are over five feet in the air, they will still go slack.

We are using 4’ long sections of pipe to support the vehicle and blow out on liftoff, which is working fine.

We did the little hop tests with 10 gallons of fuel, and rather more lox than necessary based on weighing the vehicle during loading. If we go to depletion, we want it to go to gox instead of throwing burning ethanol all over the place. We are using 200 psi pressurization, which is about what we would have remaining at the end of a fully loaded flight starting from 400 psi.

When we pumped all the fuel into the center fitting with our small peristaltic pump, we found that it mostly went into one tank. When we previously tested it with high flow loading of water it went pretty evenly, but the slow loading didn’t work well. We put quick-connect fittings on the bottom of each fuel tank and loaded five gallons into each tank independently.

The lox seems to flow in evenly with the moderate pressure loading. We are currently using around 40 psi to push the lox in, but we will probably increase that when we are going for full loads..

When we pressurized, one of the new fuel quick connect fittings started leaking. We threw a plugged socket on it as a temporary measure. This reinforces one of my biggest reliability mottos: “beware things with springs”. Check valves, quick connects, solenoids, relief valves, and regulators. We have seen them all fail at one time or another.

Ignition and liftoff was fine, but the vehicle started tilting. The gimbal went to maximum displacement trying to catch it, but it still slid to the limit of the tether, which caused a tilt shutdown.

The gimbal movement as a result of flight control gains was capped at +/-20% for no particularly good reason, so I increased that to +/-40%. The linear actuators don’t have internal limit switches, so I do want to keep them at least a little bit away from their stops.

When we drained the remaining fuel out, it was all in one tanks. It wasn’t clear how much was before/during flight, and how much was from swinging on the tether.

Hovering is only 70 psi chamber pressure, but with a mass ratio of 4, that is about where we expected it when propellant is almost depleted. Engine efficiency is very poor. This particular engine isn’t very good even at higher pressure, and at this low pressure it pretty much sucks. It could still make the 90 second flight, but we know we need to do better for the 180 second flight.

While increasing the gimbal range would probably let it fly fine as is, we wanted to try to get it to fly with the gimbal in the center of the range, so we have the maximum amount of margin both ways. We weren’t sure how much of the angle problem was due to propellant transfer moving the CG versus the initial gimbal angle not being set right, so we took steps to improve both.

We permanently mounted bubble levels on the vehicle for use during loading, and also as a reference for re-centering the gimbals. There was a fair amount of slop in the gimbal assembly when we centered everything before, so we could have been off in our centering.

The remaining slop in the engine assembly is now in just the heavy duty main thrust u-joint, which we aren’t going to try and do anything about.

Quad Hop 2
Lifted off straighter, but went into a roll, triggering a roll abort and a soft auto-land.

The roll thruster had come on full blast, but it didn’t seem to be able to stop it. The vehicle still flew at a tilt, so we assumed that the CG was still shifting, and the gimbal motion was generating roll torques.

We decided to put a valve between the tank ullages so they could be completely separated after loading, which would pull propellant back into the original tank if it temporarily transferred out. It would also be somewhat self-correcting for minor consumption differences between the tanks, because if one tank drains faster, its ullage pressure will decrease, and the opposite tank will drain faster. We moved the roll thrusters around a bit so that each direction pulled equally from a single pair of tanks. If we had a continuous roll torque, it would mean that our mixture ratio would change, and we wouldn’t get even depletion, but that wasn’t high on my list of worries.

Because the vehicle hopped up a little faster than I would have liked for testing under the lift, I changed the hover velocity profile to go slower for the first couple meters.

I was also making some software changes based on simulating the full Lunar Lander Challenge flight profiles. Several GUI changes were made, and the dead-man switch was changed to an explicit shutdown button. Holding a trigger is fine for 15 second flights, but not something you want to do for 180 seconds. During one of the simulator test flights, my finger slipped off the trigger, causing the vehicle to shutdown and drop from 50 meters. If that was a real flight, I would have felt like jumping off a bridge. Loss of telemetry is still an automatic abort, so switching to the explicit shutdown doesn’t give us any new failure modes.

Quad Hop 3
We found that when pressurizing, the extra pressure drop of the isolation valve (while open) on the ullage connecting pipe caused all of the propellant to get pushed over to that side. It only takes 2 psi of pressure differential to force liquid up the 4’ siphon tube and back over to the other tank. We were able to get it even again by closing the isolation valve and venting a little bit of pressure out of the opposite side. Once the fuel pipe was full, it would gravity level. This is still clearly not a good solution.

Liftoff was gentler this time, but we still had roll abort / auto-land. We noticed that a recent code change had made the gimbal not auto-center until engine ignition, but that didn’t have any impact on the flight. I fixed that quickly.

I was beginning to think that there might be a real flaw in our configuration, but we decided to boost our roll torque to see if we could get a short level flight, even if it had to continuously fight a roll torque.

We moved the roll thrusters to posts at the tank girth weld instead of the top of the tank, for a 50% torque increase. We drilled out the roll thrusters from 3/16″ to 1/4″ for some additional torque (the solenoids are only 1/4″, so it won’t be a full 16/9 increase).

As this work was going on, I was going back over all the data, and I realized that the first flight, which didn’t roll abort, didn’t fire the roll thrusters at all. The continuously increasing roll rate on the other hops implied a nice continuous torque, which didn’t seem to be related to the gimbal angles, which moved around.

It looked rather like the roll thrusters were backwards.

They were hooked up as they were documented to be, and as the manual test wanted them, but it turned out that the manual test (hat left / hat right on the joystick) was backwards. This was our problem. The roll thruster changes were probably still a good thing to do.

As I was working through all this, I realized two positive things about our current configuration. A conventional gimbal below the CG has a couple problems that aren’t immediately obvious: There is a control inversion when a leg hits the ground, causing gimbal angles to do the opposite of what they do when the vehicle is in the air. When you want to move to the side, you need to point the gimbal in the opposite direction to rotate the vehicle to point the direction you want to go, causing a temporary movement in the opposite direction. Both of these issues are gone with the gimbal point above the CG, which should make this vehicle a much better VTVL than our previous configurations.

This type of divergence is usually a simple matter to fix with a gain change, and I was able to replicate the behavior in the simulator by tripling the polar moment of the simulated vehicle, which makes sense comparing the quad to last year’s X-Prize Cup vehicle. Correcting the behavior in the simulator was just a matter of doubling the angular position gain (and leaving the rate gain alone).

The vehicle was still flying at a tilt, even though it was holding position. It turns out that in the process of bumping the propellant back across the siphon tubes to equalize them, we had left one of the equalization valves open for the flight. The funny thing is that as we were planning to break the ullages apart, I had commented that an isolation valve could be left in the wrong position, while requiring completely separate connectors on the ground support equipment would make that impossible.

I was feeling great about it at that point, but Matt noticed that we had burned through the injector face on the engine.

We took off the ullage connecting pipes completely, moving the pressure transducers and purge valves off to the side tanks. We made an exactly equal T split on the ground support pressurization hose, which does indeed seem to allow us to pressurize without pushing propellant from one tank to the other. We seem to have this resolved.

The drip guards over the shocks were not effective, they still rusted up from the lox frost. Water may be wicking water up from the ground when they sit in puddles. We are going to try using some synthetic grease on the shocks.

The engine injector that had the burn-through was a like-impinging, broad-fan-impinging design. It appeared that some fuel was getting past the lox fans and burning in the central recirculation zone.

We tested a new engine on Saturday that was a similar design with 38 doublets each in fuel and lox (already machined before we had the burn through), but with a machining change to allow the fan impingement points to be much closer together, and the holes were all reamed, 0.0650” for fuel, 0.0715” for lox. The film cooling holes were changed from 10 x 1/16” holes to 19 x 1/32”, so the total film cooling mass flow was about halved, but more evenly spread. Another modest change we made was going to a square igniter / engine mount post instead of a round one, which has some fabrication and repeatability benefits.

The engine had better chamber pressure for a given feed pressure than the one that burned through, implying better combustion, but it ran rather rougher. The film cooling still seemed more than sufficient. We are going to go ahead and hop the vehicle on this engine to test out the gain and loading modifications, but it won’t be the final engine, and it may well burn through just like the last one did.

My theory is that the greater roughness is due to the machining that brought the impinging exits closer together shortening the L:D of one leg of the impinging injectors to only 2:1. I will probably be making another test with a changed internal contour that allows us to make all the holes at least a 4:1 aspect ratio.

I also started machining an injector out of 303 stainless steel. Depending on the conditions, this may either prevent the burn through, or burn through faster. I haven’t decided which hole pattern to try with it yet. I was pleased to find that I could run the same CNC program I use for aluminum, and just cut the speed and feed down to 20% to machine stainless. Hopefully the injector holes won’t be any more troublesome. We may wind up going with copper for the injector if we continue to have heat problems, but we prefer to weld stainless over brazing copper, and copper is a good deal more expensive.